UMA (Unlicensed Mobile Access) technology allows mobile and fixed clients to communicate with each other through nearby wireless access points. The mobile and fixed clients typically communicate with the access points using unlicensed frequencies, although a wired connection to an access point may also be used. The access points typically communicate with a larger telecommunications infrastructure using wired connections, such as an Internet connection, or using licensed frequencies, such those used for cellular telephony. In some implementations of UMA, a MSC (Mobile Switching Center) acts as an anchor for a call between two (or more) UMA clients. One or more media gateways are used to carry the call's traffic between the clients.
The MG (Media Gateway) of a mobile communications system has transcoding equipment capable of changing voice or data encoding or formatting. In, for example, a GSM base station subsystem, specialized media gateways (i.e., transcoder and rate adaptation units) are used to convert voice traffic from the coding and format of the cellular telephone to the PCM (Pulse Code Modulation) TDM (Time Division Multiplexed) format of the landline network. While this works well for mobile cellular telephones, it may introduce transcoding that is unnecessary for other types of terminals by first converting all voice traffic to the PCM TDM format of the network.
When two clients capable of running the same codec establish a VoIP call through a GSM system, the transcoding functions of the MG are not needed. Running the transport path through the MG consumes resources of the MG that may be applied to other calls. Any additional transcoding applied by the MG may also reduce the quality of the transport packets.